Programmable logic circuit and nonvolatile fpga

ABSTRACT

A programmable logic circuit includes: first to third wiring lines, the second wiring lines intersecting with the first wiring lines; and cells provided in intersecting areas, at least one of cells including a first transistor and a programmable device with a first and second terminals, the first terminal connecting to one of a source and a drain of the first transistor, the second terminal being connected to one of the second wiring lines, the other of the source and the drain being connected to one of the first wiring lines, and a gate of the first transistor being connected to one of the third wiring lines. One of source and drain of each of the first cut-off transistors is connected to the one of the second wiring lines, and an input terminal of each of first CMOS inverters is connected to the other of the source and the drain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2014-012695 filed on Jan. 27, 2014 in Japan, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to programmable logic circuits and nonvolatile field programmable gate arrays (FPGAs).

BACKGROUND

Reconfigurable integrated circuits, notably field programmable gate arrays (FPGAs), have received attention in recent years. FPGAs realize basic logic data by means of logic blocks. Users can achieve desired logic functions by switching connections among logic blocks by means of switch blocks. Configuration memories store logic data of the logic blocks and data of the switch blocks for changing connections. Desired logic functions can be realized based on the stored data.

Nonvolatile FPGAs can be constituted by storing nonvolatile data in the configuration memories. Examples of nonvolatile FPGAs include those employing antifuse devices that are typical programmable devices. In these nonvolatile FPGAs, the switch blocks connecting the logic blocks are replaced with antifuse devices. An antifuse device, however, requires a high voltage in a write operation. This reduces the operational speed of conventional antifuse FPGAs, since signals on high voltage applied wiring lines cannot be directly amplified by low voltage driven CMOS circuits capable of operating at a high speed. If signals on high voltage applied wiring lines can be directly amplified by the low voltage driven CMOS circuits, gate insulating films of transistors in the CMOS circuits may be broken down. Nonvolatile FPGAs including antifuse devices also have a problem of not capable of employing a multiple memory architecture, in which a plurality of memories are connected to a switch and read depending on applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a field programmable gate array (FPGA).

FIG. 2 is a circuit diagram showing a first example of a programmable logic circuit according to a first embodiment.

FIG. 3 is a circuit diagram showing a second example of the programmable logic circuit according to the first embodiment.

FIG. 4 is an explanatory diagram illustrating a write operation of the programmable logic circuit according to the second example.

FIG. 5 is an explanatory diagram illustrating a write operation of the programmable logic circuit according to the second example.

FIG. 6 is a circuit diagram showing a third example of the programmable logic circuit according to the first embodiment.

FIG. 7 is an explanatory diagram illustrating a read operation of the programmable logic circuit according to the second example.

FIG. 8 is a circuit diagram showing a fourth example of the programmable logic circuit according to the first embodiment.

FIGS. 9A to 9C are cross-sectional views showing examples of a programmable device.

FIG. 10 is a circuit diagram showing a programmable logic circuit according to a second embodiment.

FIG. 11 is an explanatory diagram illustrating a write operation of the programmable logic circuit according to the second embodiment.

FIG. 12 is an explanatory diagram illustrating a write operation of the programmable logic circuit according to the second embodiment.

FIG. 13 is an explanatory diagram illustrating an FPGA operation of a multi-context FPGA including the programmable logic circuit according to the second embodiment.

FIGS. 14A and 14B are circuit diagrams illustrating a first example and a second example of a programmable logic circuit according to a third embodiment.

FIGS. 15A and 15B are circuit diagrams illustrating a first example and a second example of a programmable logic circuit according to a fourth embodiment.

FIG. 16 is a cross-sectional view showing an example of a programmable device included in a programmable logic circuit according to a fifth embodiment.

FIG. 17 is a circuit diagram showing an example of a programmable logic circuit according to a sixth embodiment.

FIG. 18 is a diagram showing a circuit configuration of the sixth embodiment for causing the bootstrap effect.

FIG. 19A is a diagram showing a circuit used for a simulation relating to the bootstrap effect.

FIG. 19B is a diagram showing simulation results relating to the bootstrap effect.

FIG. 20 is a circuit diagram showing an example of a programmable logic circuit according to a modification of the sixth embodiment.

FIG. 21 is a circuit diagram showing an example of a programmable logic circuit according to a seventh embodiment.

FIG. 22 is a circuit diagram showing an example of a programmable logic circuit according to an eighth embodiment.

FIG. 23 is an explanatory circuit diagram illustrating a write method of the programmable logic circuit according to the eighth embodiment.

FIG. 24 is an explanatory circuit diagram illustrating an operation method of the programmable logic circuit according to the eighth embodiment.

FIG. 25 is a circuit diagram showing an example of a programmable logic circuit according to a ninth embodiment.

FIG. 26 is an explanatory circuit diagram illustrating a write method of the programmable logic circuit according to the ninth embodiment.

FIG. 27 is an explanatory circuit diagram illustrating an operation method of the programmable logic circuit according to the ninth embodiment.

FIG. 28 is a circuit diagram showing an example of a programmable logic circuit according to a modification of the ninth embodiment.

FIG. 29 is a circuit diagram of a programmable logic circuit according to a modification of the eighth embodiment.

DETAILED DESCRIPTION

A programmable logic circuit according to an embodiment includes: a plurality of first wiring lines; a plurality of second wiring lines intersecting with the first wiring lines in intersecting areas; a plurality of third wiring lines; a plurality of cells provided in the intersecting areas, at least one of the cells including a first transistor with a source, a drain, and a gate, and a programmable device with a first terminal and a second terminal, the first terminal connecting to one of the source and the drain of the first transistor, the second terminal being connected to one of the second wiring lines, the other of the source and the drain of the first transistor being connected to one of the first wiring lines, and the gate of the first transistor being connected to one of the third wiring lines; a plurality of first cut-off transistors each including a source and a drain, one of the source and the drain being connected to the one of the second wiring lines; a plurality of first CMOS inverters corresponding to the first cut-off transistors, each of the first CMOS inverters including an input terminal, the input terminal being connected to the other of the source and the drain of the corresponding one of the first cut-off transistors.

Embodiments will now be explained with reference to the accompanying drawings.

First Embodiment

Before a programmable logic circuit according to a first embodiment is described, a configuration of general FPGAs will be described. As shown in FIG. 1, an FPGA 100 generally includes a plurality of basic blocks 110 arranged in an array form. Each basic block 110 is connected to adjacent basic blocks 110 with wiring lines, and includes a logic block 120 and a switch block 130. The logic block 120 performs logical operations basically using a look-up table containing a truth table. Each switch block 130 controls the connection and the disconnection of the wiring lines connecting to adjacent basic blocks 110 so that signals are transmitted to given directions. Each switch block 130 also connects to the logic block 120 included in the relevant basic block 110 including the switch block 130. The logic block 120 and the switch block 130 are capable of controlling the connection based on data stored in a configuration memory of the programmable logic circuit.

A first example of the programmable logic circuit according to a first embodiment will be described below with reference to FIG. 2. The programmable logic circuit 140 according to the first example is used in switch blocks 130 of FPGAs, and includes at least one source line SL, at least one word line WL, at least one bit line BL crossing the source line SL, at least one cell 10, a CMOS inverter 20 for driving the source line SL, a cut-off transistor 32 connecting to the bit line BL, and a CMOS inverter 34 connecting to the bit line BL via the cut-off transistor 32. The CMOS inverter 20 can be eliminated if the source line SL can be driven by an element other than the CMOS inverter 20.

The cell 10 is disposed in an intersection region between the source line SL and the bit line BL, and includes a selection transistor 12 and a programmable device 14. The gate of the selection transistor 12 is connected to the word line (selection signal line) WL, and one of the source and the drain is connected to the source line SL. One terminal of the programmable device 14 is connected to the other of the source and the drain of the selection transistor 12, and the other terminal is connected to the bit line BL.

One selection transistor 12 is connected in series to each programmable device 14 in the first embodiment. If a write operation is performed on the programmable device 14, or the cell 10 is used in an FPGA operation, a voltage to turn ON the selection transistor 12 is applied to the gate thereof. The voltage applied is typically a power supply voltage Vdd if the selection transistor 12 is an n-channel MOS transistor. Although the selection transistor 12 and the cut-off transistor 32 are illustrated as n-channel MOS transistors in FIG. 2, they may be p-channel MOS transistors.

The programmable device 14 is generally programmed by a program voltage Vpp that is higher than the power supply voltage Vdd used in a logic operation. The CMOS inverters 20, 34, the selection transistor 12, and the cut-off transistor 32, however, are preferably formed of usual transistors that operate with the power supply voltage Vdd. Although high voltage transistor, which operates with the program voltage Vpp, is used in usual, it decreases FPGA operation performance and increases the circuit area. The use of usual transistors operating with the power supply voltage Vdd may allow FPGAs including the programmable devices 14 to be formed without increasing the circuit area.

FIG. 3 shows a second example of the programmable logic circuit according to the first embodiment. The programmable logic circuit 140 according to the second example includes a 2×2 array (matrix) structure with four cells 10 shown in FIG. 2, and may be used for both switch blocks 130 and logic blocks 120 of FPGAs. The programmable logic circuit 140 according to the second example includes two source lines SL₁, SL₂, two word lines WL₁, WL₂, two bit lines BL₁, BL₂ crossing the source lines SL₁, SL₂, the four cells 10 _(ij) (i, j=1, 2), CMOS inverters 20 _(j) each driving one of the source lines SL_(j) (j=1, 2), cut-off transistors 32 _(i) each connecting to one of the bit lines BL_(i) (i=1, 2), and CMOS inverters 34 _(i) each connecting to the corresponding bit line BL_(i) via the corresponding cut-off transistor 32 _(i).

The cells 10 _(i1), 10 _(i2) arranged in the same row (i (i=1, 2)) share the corresponding word line WL_(i) and the corresponding bit line BL_(i), and the cut-off transistor 32 _(i) and the CMOS inverter 34 _(i) in the same row (i (i=1, 2)) share the corresponding bit line BL_(i). The cells 10 _(1j), 10 _(2j) arranged in the same column (j=1, 2) share the corresponding source line SL_(j) and the corresponding CMOS inverter 20 _(j). The circuit area can be reduced with such a configuration.

A write operation of the cell 10 _(ij) (i, j=1, 2) arranged in an array (matrix) will be described with reference to FIGS. 4 and 5. FIG. 4 shows a 2×2 matrix. The same operation can be performed by a 1×2 matrix, a 1×3 matrix, a 2×1 matrix, a 3×1 matrix, a 2×3 matrix, a 3×3 matrix, or any other matrix including more cells. Although n-channel MOS transistors are used as selection transistors and cut-off transistors in the descriptions with reference to FIGS. 4 and 5, p-channel MOS transistors may also be used to perform the same operation if the voltage Vdd is replaced with a reference voltage Vss, which is lower than Vdd.

FIG. 4 shows the state where all the programmable devices 14 in the cells 10 _(ij) (i, j=1, 2) are fresh (virgin), and a write operation is performed on the programmable device 14 of the cell 10 ₁₁. A voltage Vdd to turn ON the selection transistor 12 in the cell 10 ₁₁ is applied to the word line WL₁ to which the selection transistor 12 of the cell 10 ₁₁ is connected. A program voltage Vpp is applied to the bit line BL₁ to which the programmable device 14 of the cell 10 ₁₁ is connected, and a reference potential Vss (usually the ground potential) is applied to the source line SL₁ to which the selection transistor 12 of the cell 10 ₁₁ is connected. The voltage Vdd is also applied to all of the other wiring lines and terminals, for example the word line WL₂, the bit line BL₂, the source line SL₂, and the gates of the cut-off transistors 32 ₁, 32 ₂. As a result, a voltage Vpp−Vss is applied to the programmable device 14 of the cell 10 ₁₁ to break down the programmable device 14 to make it conductive. Data is written to the programmable device 14 of the cell 10 ₁₁ in this manner.

No data is written to the programmable devices 14 of the other cells 10 ₁₂, 10 ₂₁, 10 ₂₂. For example, the selection transistor 12 of the cell 10 ₁₂ in the OFF state since the voltage Vdd is applied to both the gate and the source. As a result, although the voltage Vpp is applied to one terminal of the programmable device 14 in the cell 10 ₁₂, the voltage Vpp−Vss is not applied across the programmable device 14. Therefore, the programmable device 14 is not broken. Only the voltage Vdd−Vss is applied across the programmable device 14 of the cell 10 ₂₁, and therefore the programmable device 14 thereof is not broken. The voltage Vdd is applied to both the gate and the source of the selection transistor 12 of the cell 10 ₂₂. As a result, the selection transistor 12 thereof is in the OFF state. Although the voltage Vpp is applied to the drain of the cut-off transistor 32 ₁ connected to the bit line BL₁, to which the cell 10 ₁₁ and the cell 10 ₁₂ are connected, the voltage Vdd is applied to the gate thereof. Assuming that the threshold voltage of the cut-off transistor 32 ₁ is Vth_c, the cut-off transistor 32 ₁ is turned OFF if the potential at the source thereof connecting to the CMOS inverter 34 ₁ becomes Vdd−Vth_c, to prevent the voltage Vpp from being applied to the input terminal of the CMOS inverter 34 ₁. This also prevents the breakdown of the CMOS inverter 34 ₁.

The program voltage Vpp breaks the programmable device 14. An I/O voltage Vio for use in an input circuit may be used as the program voltage Vpp. This may prevent an increase in the circuit area since no power supply circuit should be newly added.

The upper limit of the program voltage Vpp is set so that the difference Vpp−Vdd between the voltages applied to the cut-off transistors 32 ₁, 32 ₂ would not break down the cut-off transistors 32 ₁, 32 ₂. The breakdown of the gate insulating film is time dependent, and should meet the following formula:

$\frac{{Vpp} - {Vdd}}{Tox} < {Ebk}$

where Ebk is the electric field at which the gate insulating film is broken in a certain programming time, and Tox is the thickness of the gate insulating film. The above formula can be translated into:

Vpp<Ebk×Tox+Vdd.

The electric field Ebk required to perform programming in a few tens microseconds is about 20 MV/cm. This requires the program voltage Vpp to meet

Vpp<2.0×10⁹ ×Tox+Vdd.

Since the program voltage Vpp is greater than the voltage Vdd,

Vdd<Vpp<2.0×10⁹ ×Tox+Vdd.

For example, if Tox=5 nm, and Vdd=1.8V,

1.8V<Vpp<11.8V.

Thus, the program voltage Vpp can write data to the programmable device 14, but do not break down the gate insulating film of the transistor. This allows the wiring line to which the program voltage Vpp is applied to be directly amplified by the CMOS circuit, and the FPGA to operate at a high speed.

FIG. 5 is an explanatory diagram illustrating an operation to write data the programmable device 14 of the cell 10 ₁₂ after the cell 10 ₁₁ is programmed. A program voltage Vpp is applied to the bit line BL₁ to which the cell 10 ₁₁ and the cell 10 ₁₂ are connected, a voltage Vdd is applied to the word line WL₁, and a voltage Vss is applied to the source line SL₂ to which the cell 10 ₁₂ is connected. The voltage Vdd is also applied to the word line WL₂, the source line SL₁, and the bit line BL₂ to which the cell 10 ₂₁ is connected. The programmable device 14 in the cell 10 ₁₁ is already conductive. However, since the voltage Vdd is applied to the gate and the source of the selection transistor 12 of the cell 10 ₁₁, the selection transistor 12 is in the OFF state. This prevents the sneak current caused by the program voltage Vpp to be applied to the other cells 10 ₁₂, 10 ₂₁, 10 ₂₂. A voltage difference Vpp−Vss is applied across the programmable device 14 of the cell 10 ₁₂ and breaks it. This allows data to be written to the programmable device 14 of the cell 10 ₁₂. How the cells 10 ₂₁, the cells 10 ₂₂, the bit lines BL₁, BL₂, and the CMOS inverters 34 ₁, 34 ₂ are protected is the same as that in the case shown in FIG. 4.

FIG. 6 shows a third example of the programmable logic circuit according to the first embodiment. The programmable logic circuit 140A according to the third example can be used for both the switch blocks 130 and the logic blocks 120 of FPGAs, and obtained by disposing cut-off transistors 22 ₁, 22 ₂ at the output sides of the CMOS inverters 20 ₁, 20 ₂ connecting to the source lines SL₁, SL₂, respectively, in the programmable logic circuit 140 according to the second example shown in FIG. 3.

As described with reference to FIGS. 4 and 5, basically the program voltage Vpp, which is high, is not applied to the source lines SL₁, SL₂. The program voltage Vpp, however, may be applied to the source line SL₁ immediately after the breakdown of the programmable device 14 of the cell 10 ₁₁ is caused in FIG. 4. In actual cases, the voltage to be applied to the source line SL₁ may not become very high since the voltage is determined by the resistances of the broken programmable device 14, the selection transistor 12, and the n-channel MOS transistors or p-channel MOS transistors of the CMOS inverter 20 ₁. If it is expected that a high voltage may be applied to the source line in view of the resistance of the broken programmable device and variations in resistance of the aforementioned transistors, the cut-off transistors 22 ₁, 22 ₂ are preferably disposed in order to prevent the breakdown of the CMOS inverters 20 ₁, 20 ₂ connected to the source lines SL₁, SL₂. Like the other devices, the voltage Vdd is also applied to the gates of the cut-off transistors 22 ₁, 22 ₂ in operation. This reliably prevents the breakdown of the CMOS inverters 20 ₁, 20 ₂ connected to the source lines SL₁, SL₂.

A read operation of the programmable logic circuit 140 according to the second example shown in FIG. 3 will be described with reference to FIG. 7. The selection transistor 12 and the cut-off transistors 32 ₁, 32 ₂ are in the ON state. If the selection transistor 12 and the cut-off transistors 32 ₁, 32 ₂ are n-channel MOS transistors, the voltage Vdd may be applied to the gates. However, the values of operational signals of the FPGA may be reduced by the threshold voltage in this case to delay the operational speed. A read voltage Vread, which is higher than the voltage Vdd, is preferable since the FPGA operates without being delayed. The Vread can be expressed by the following formula if Vth_s=Vth_c:

Vread>Vdd+Vth_c=Vdd+Vth_s

where Vth_s is the threshold voltage of the selection transistor 12, and Vth_c is the threshold voltage of the cut-off transistors 32 ₁, 32 ₂. If Vth_s differs from Vth_c, the higher voltage is employed. For example, if Vth_s>Vth_c,

Vread>Vdd+Vth_s.

FIG. 8 shows a fourth example of the programmable logic circuit according to the first embodiment. The programmable logic circuit 140B of the fourth example is included in logic blocks 120 of FPGAs, and has a configuration with four cells 10 shown in FIG. 2 arranged in a 2×2 array. The programmable logic circuit 140B according to the fourth example includes two source lines SL₁, SL₂, two word lines WL₁, WL₂, two bit lines BL₁, BL₂ crossing the source lines SL₁, SL₂, four cells 10 _(ij), (i, j=1, 2), and cut-off transistors 32 _(i) each connecting to a corresponding bit line BL_(i) (i=1, 2).

The programmable logic circuit 140B according to the fourth example programs one of the programmable devices 14 connected to each bit line BL_(i) (i=1, 2). By applying a voltage Vdd to one of the two source line SL₁, SL₂ and a voltage Vss to the other, the voltage Vdd or Vss can be outputted from the bit lines BL₁, BL₂ depending on the state of the programmable devices 14. The subsequent operations are performed using the voltage Vdd and the voltage Vss like other logic circuits.

FIGS. 9A to 9C are cross-sectional views showing examples of the programmable device 14. FIG. 9A is a cross-sectional view showing a first example in which the programmable device 14 is a MOS transistor, and the write operation is performed by breaking down the gate insulating film of the MOS transistor. The programmable device 14 according to the first example includes a source 42 a and a drain 42 b disposed in a semiconductor layer 40 so as to be separated from each other, a gate insulating film 44 disposed on the semiconductor layer 40 between the source 42 a and the drain 42 b, the gate insulating film 44 partially overlapping the source 42 a and the drain 42 b, and a gate 46 disposed on the gate insulating film 44. The breakdown of the gate insulating film 44 is performed in a region where the gate 46 overlaps the source 42 a by applying the program voltage Vpp to the gate 46 and the voltage Vss to the source 42 a. Only the source side may be broken by setting the drain 42 b to be in a floating state. This may reduce the circuit area since the number of wiring lines becomes the smallest in this case. Shallow Trench Isolation (STI) may be performed on the drain terminal that is not used.

An identical potential may be applied to both the source 42 a and the drain 42 b so that the insulating film 44 may be broken down in at least one of the regions where the insulating film 44 overlaps the source 42 a and the drain 42 b. This allows two overlapping regions to be broken down, thereby increasing the breakage probability and shortening the write time. This MOS transistor may be used as the cut-off transistor 32 and the CMOS inverter 34, which are driven by Vdd, and the thickness of the gate oxide film in this MOS transistor may be similar to that of the transistor used as the cut-off transistor 32 and the CMOS inverter 34 (with variations of about ±20%). Therefore, no special device should be prepared as the programmable device, and the number of processes and the manufacturing costs can be reduced.

An identical potential may be applied to the source 42 a, the drain 42 b, and the semiconductor layer 40. The number of wiring lines may be increased in this case, but the write time can be considerably reduced. The transistor in this case can be either an n-channel MOS transistor or a p-channel MOS transistor. The semiconductor layer 40, however, should be electrically isolated from adjacent devices if the gate 46 and the semiconductor layer 40 may be electrically connected to each other since FPGA signals pass through the broken and conducting path in the FPGA according to the first embodiment. Therefore, the terminal of the semiconductor layer 40 is preferably nonconductive (in the floating state). Alternatively, the reference voltage Vss is preferably applied to the terminal of the semiconductor layer 40 in the case of the n-channel MOS transistor, and the power supply voltage Vdd, the program voltage Vpp, or an intermediate voltage is preferably applied thereto in the case of the p-channel MOS transistor.

FIG. 9B shows a cross-sectional view of a second example, in which the programmable device 14 employs a pn junction, and a high reverse-bias voltage is applied to break down the pn junction to perform a write operation. The programmable device 14 according to the second example includes an n well 52 formed in a semiconductor layer 50, and a p well 54 formed in the n well 52. The program voltage Vpp is applied to the n well 52, and the voltage Vss is applied to the p well 54. The pn junction may also be formed by forming n well 52 in the p well 54.

FIG. 9C shows a cross-sectional view of a third example, in which the programmable device 14 employs a pn junction of polycrystalline silicon, and a high reverse-bias voltage is applied to break down the pn junction to perform a write operation. The programmable device 14 according to the third example includes an insulating film 62 disposed on a semiconductor layer 60, and an n layer 64 and a p layer 66 of polycrystalline silicon disposed on the insulating film 62. The n layer 64 and the p layer 66 are formed by introducing an n-type impurity and a p-type impurity to the gate of polycrystalline silicon in the MOS transistor. The junction region of the pn junction does not include silicide. Thus, the pn junction may be broken down by a high reverse-bias voltage. The pn junction of polycrystalline silicon can be made smaller than a pn junction with wells formed in a semiconductor layer.

As described above, according to the first embodiment, a voltage for writing data to a programmable device and not breaking down a gate insulating film of a transistor can be selected as the program voltage Vpp. This allows direct amplification of signals in wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

Second Embodiment

FIG. 10 shows a programmable logic circuit according to a second embodiment. The programmable logic circuit 140C according to the second embodiment is obtained by replacing the cells 10 ₁₁-10 ₂₂ of the programmable logic circuit 140 shown in FIG. 3 with cells 10A₁₁-10A₂₂. Each cell 10A_(ij) (i, j=1, 2) includes four selection transistors 12 a, 12 b, 12 c, 12 d and two programmable devices 14 a, 14 b. The selection transistors 12 a, 12 b and the programmable device 14 a is connected in series and the selection transistors 12 c, 12 d and the programmable device 14 b are connected in series in each cell 10A_(ij) (i, j=1, 2). Specifically, one of the source and the drain of the selection transistor 12 a is connected to a source line SL_(j), and the other is connected to one of the source and the drain of the selection transistor 12 b, the other of the source and the drain of the selection transistor 12 b is connected to one terminal of the programmable device 14 a, and the other terminal of the programmable device 14 a is connected to a bit line BL_(i) in each cell 10A_(ij) (i, j=1, 2). One of the source and the drain of the selection transistor 12 c is connected to the source line SL_(j), the other is connected to one of the source and the drain of the selection transistor 12 d, the other of the source and the drain of the selection transistor 12 d is connected one terminal of the programmable device 14 b, and the other terminal of the programmable device 14 b is connected to the bit line BL_(i) in each cell 10A_(ij) (i, j=1, 2).

The gate of the selection transistor 12 a is connected to a word line WL1 ₁, and the gate of the selection transistor 12 b is connected to a word line WL2 ₁ in each cell 10A_(ij) (j=1, 2). The gate of the selection transistor 12 c is connected to a word line WL3 ₁ and the gate of the selection transistor 12 d is connected to a word line WL4 ₁ in each cell 10A_(1j) (j=1, 2). The gate of the selection transistor 12 a is connected to a word line WL1 ₂, and the gate of the selection transistor 12 b is connected to the word line WL2 ₂ in each cell 10A_(2j) (j=1, 2). The gate of the selection transistor 12 c is connected to a word line WL3 ₂ and the gate of the selection transistor 12 d is connected to a word line WL4 ₂ in each cell 10A_(2j) (j=1, 2).

Such a configuration achieves a multi-context FPGA capable of switching from a circuit with a certain logic function to another circuit with another logic function instantaneously by switching between the programmable devices in a cell. The example here shows that each cell 10A_(ij) (i, j=1, 2) includes the two programmable devices 14 a, 14 b. A multi-context FPGA capable of writing more circuit information can be achieved by connecting more than two programmable devices in parallel. The circuit information can also be switched dynamically. The integration of the FPGA can be effectively improved by the multi-context configuration.

The write operation of the programmable device according to the second embodiment will be described with reference to FIG. 11. The breakdown of the programmable device 14 a in the cell 10A₁₁ will be described as an example. A write voltage (program voltage) Vpp is applied to the bit line BL₁ to which the programmable device 14 a is connected, a voltage Vdd is applied to the gates of the two selection transistors 12 a, 12 b to which the programmable device 14 a is connected, and a voltage Vss is applied to the source line SL₁. The voltage Vss is also applied to the gate of the selection transistor 12 c connected to the programmable device 14 b of the cell 10A₁₁, and the voltage Vdd is also applied to the gate of the selection transistor 12 d. It should be noted that the voltage Vss is applied to the selection transistor 12 c connected to the source line SL₁. The voltage Vdd is also applied to the word lines WL1 ₂, WL2 ₂, WL3 ₂, WL4 ₂, the bit line BL₂, and the source line SL₂. The voltage Vdd is also applied to the gates of the cut-off transistors 32 ₁, 32 ₂. As a result, a voltage difference Vpp−Vss is applied across the programmable device 14 a to break down the programmable device 14 a. The programmable device 14 b is not broken down since the voltage from the source line SL₁ is blocked by the two selection transistors 12 c, 12 d. The reason why the programmable devices 14 a, 14 b and the CMOS inverters 34 ₁, 34 ₂ in the other cells 10A₁₂, 10A₂₁, 10A₂₂ are not broken down is the same as that for the first embodiment, and will not be described repeatedly.

The write operation for the programmable device 14 b in the cell 10A₁₁ after the programmable device 14 a is broken down will be described with reference to FIG. 12. The write voltage Vpp is applied to the bit line BL₁, the voltage Vss is applied to the word line WL1 ₁, and the voltage Vdd is applied to the word lines WL2 ₁, WL3 ₁, WL4 ₁, WL1 ₂, WL2 ₂, WL3 ₂, WL4 ₂, the bit line BL₂, and the source line SL₂. The voltage Vdd is also applied to the gates of the cut-off transistors 32 ₁, 32 ₂. The programmable device 14 b of the cell 10A₁₁ is broken down by the voltage Vpp−Vss applied thereto. The write voltage Vpp is also applied to one terminal of the programmable device 14 a in the cell 10A₁₁. This write voltage Vpp, however, is conveyed to the drain terminal of the selection transistor 12 b since the programmable device 14 a has already been broken. Since the voltage Vdd is applied to the gate of the selection transistor 12 b, a voltage Vpp−Vdd is applied between the gate and the drain of the selection transistor 12 b. Therefore, the selection transistor 12 b is not broken down. Assuming that the threshold voltage of the selection transistor 12 b is Vthc, a voltage Vdd−Vthc is applied to the drain terminal of the selection transistor 12 a. Since the voltage Vss is applied to the gate of the selection transistor 12 a, a voltage Vdd−Vthc−Vss is applied between the gate and the drain of the selection transistor 12 a. As a result, the selection transistor 12 a is not broken down. The reason why the other elements of the cells 10A₁₂, 10A₂₁, 10A₂₂ and the CMOS inverters 34 ₁, 34 ₂ are not broken down is the same as that for the first embodiment. A multi-context FPGA including programmable devices can be achieved in this manner without breaking down devices unnecessarily.

Multi-context FPGAs have improved integration. A multi-context FPGA includes a plurality of configuration memories (programmable logic circuits) per one FPGA circuit, and switches the configuration memories in a read operation to switch to other circuit configurations instantaneously. If the switching of the configuration memories is performed on the application basis, a plurality of FPGA chips can be effectively achieved by one chip. Furthermore, dynamic reconfiguration can be performed by switching the configuration memories dynamically. This allows the reduction in area of FPGAs.

An FPGA operation of a multi-context FPGA including the programmable logic circuit according to the second embodiment shown in FIG. 10 will be described with reference to FIG. 13. A voltage Vread is applied to the gates of the selection transistors 12 a, 12 b connected to the programmable devices 14 a for a selected context 16 to turn ON the selection transistors 12 a, 12 b. As in the case of the first embodiment, the voltage Vread may be the power supply voltage Vdd, or a voltage greater than Vdd by the threshold voltage of the selection transistor and the cut-off transistor. A voltage Vss is applied to the gate of one of the selection transistors 12 c, 12 d connected to the programmable device 14 b for the non-selected context, for example the selection transistor 12 c, to turn it OFF, and a voltage Vss or Vdd is applied to the gate of the other, for example the selection transistor 12 d. This allows signals in FPGA operation to pass through the programmable device 14 a for the selected context 16 to perform a circuit operation in accordance with the data written to the programmable device 14 a. The switching of the selected context can be performed instantaneously by changing the gate voltages applied to the selection transistors. This improves the integration of the FPGA effectively.

As described above, according to the second embodiment, a voltage for writing data to a programmable device and not breaking down the gate insulating film of a transistor can be selected as the program voltage Vpp as in the first embodiment. This allows direct amplification of signals passing through wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

Third Embodiment

A programmable logic circuit according to a third embodiment will be described with reference to FIG. 14A and 14B. The programmable logic circuit according to the third embodiment employs a MOS transistor of the type where the gate insulating film is to be broken down as the programmable device 14 of the programmable logic circuit according to the first example of the first embodiment shown in FIG. 2. FIGS. 14A and 14B are circuit diagrams showing a first example and a second example of the programmable logic circuit according to the third embodiment. The gate of a MOS transistor 15 a in the programmable logic circuit according to the first example shown in FIG. 14A is connected to a bit line BL, one of the source and the drain is connected to one of the source and the drain of a selection transistor 12, and the other of the source and the drain of the MOS transistor 15 a is connected to nowhere. The gate of a MOS transistor 15 b of the programmable logic circuit according to the second example shown in FIG. 14B is connected to a bit line BL, and both the source and the drain thereof are connected to one of the source and the drain of a selection transistor 12. The programmable device 14 basically has two terminals, and which terminals are connected to which wiring lines does not matter. However, in breaking down the gate insulating film of a MOS transistor a high potential program voltage is preferably applied to the gate thereof since if the high potential program voltage is applied to at least one of the source and the drain of the MOS transistor, the regions of the PN-junction of source and the drain connecting the semiconductor layer may possibly be broken down earlier than the gate insulating film of the MOS transistor.

As described above, according to the third embodiment, a voltage for writing data to a programmable device and not breaking down the gate insulating film of a transistor can be selected as the program voltage Vpp as in the first embodiment. This allows direct amplification of signals of wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

In the first to third embodiments, cut-off transistors 32 ₁, 32 ₂ are provided to connect to the bit lines BL₁, BL₂. Cut-off transistors 32 ₁, 32 ₂ can be provided to connect to the source lines SL₁, SL₂.

Fourth Embodiment

A programmable logic circuit according to a fourth embodiment will be described with reference to FIGS. 15A and 15B, which are circuit diagrams showing a first example and a second example of the programmable logic circuit according to the fourth embodiment.

Generally, a cell to be programmed is selected or not selected by a selection transistor connected to a word line. If it is known in advance that only one programmable device of a cell in a series of cells is to be programmed, no selection transistor may be used for the series of cells.

The programmable logic circuit according to the first example of the fourth embodiment is obtained by eliminating the selection transistor 12 from each cell of the programmable logic circuit according to the second example of the first embodiment shown in FIG. 3. Therefore, the cut-off transistors 32 ₁, 32 ₂ are disposed before the CMOS inverters 34 ₁, 34 ₂ on the output side.

The programmable logic circuit according to the second example of the fourth embodiment is obtained by eliminating the selection transistor 12 from each cell of the programmable logic circuit according to the third example of the first embodiment shown in FIG. 6. Therefore, the cut-off transistors 32 ₁, 32 ₂ are disposed before the CMOS inverters 34 ₁, 34 ₂ on the output side, and the cut-off transistors 22 ₁, 22 ₂ are disposed after the CMOS inverters 20 ₁, 20 ₂ on the input side.

The series of cells may be a row of cells, or a column of cells. If a high potential program voltage Vpp can be applied to only one programmable device 14 in a series of cells in a programming operation, it is not necessary to concern an increase in leakage current and an occurrence of sneak current during programming operation. (sneak current is a current flowing from a target cell that is in the ON state to another cell in the ON state through wiring lines). If, for example, only one of the programmable devices 14 that share a bit line, for example the bit line BL₁, to which the program voltage Vpp is applied, should be programmed, the first example or the second example of the fourth embodiment that do not include any selection transistor may be employed.

As described above, according to the fourth embodiment, a voltage for writing data to a programmable device and not breaking down the gate insulating film of a transistor can be selected as the program voltage Vpp as in the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

Fifth Embodiment

A programmable logic circuit according to a fifth embodiment will be described with reference to FIG. 16. The programmable logic circuit according to the fifth embodiment employs a programmable device 14A shown in FIG. 16 as the programmable device 14 of the programmable logic circuit according to any of the first embodiment, the second embodiment, and the fourth embodiment.

The programmable device 14A shown in FIG. 16 is a resistive change nonvolatile memory including a lower electrode 17 with one or more layers, a resistive change film 18 with one or more layers, and an upper electrode 19 with one or more layers, the lower electrode 17, the resistive change film 18, and the upper electrode 19 being stacked in this order. The resistance value of the resistive change film 18 can be changed to a high resistance value, a low resistance value, and an intermediate resistance value depending on the magnitude, the direction, and the application time of the voltage applied between the upper electrode 19 and the lower electrode 17. In order to bring an insulating film of a resistive change memory into a state where the resistance value thereof can be changed, a high voltage is generally applied to the resistive change film to introduce defects (filament) thereto. By setting the initial voltage (forming voltage) in this case to be the voltage Vpp, the operation to introduce defects can be performed in a similar manner to the operation to program the programmable device 14. Therefore, a resistive change memory can be employed as the programmable device of the programmable logic circuit according to any of the first embodiment, the second embodiment, and the fourth embodiment. The resistive change memory serving as the programmable device 14 becomes a nonvolatile memory after defects are introduced thereto. Accordingly, the FPGA can be repeatedly rewritten.

As described above, according to the fifth embodiment, a voltage for writing data to a programmable device and not breaking down the gate insulating film of a transistor can be selected as the program voltage Vpp as in the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

Sixth Embodiment

A programmable logic circuit according to a sixth embodiment will be described with reference to FIGS. 17 to 20. The programmable logic circuit according to the sixth embodiment is obtained by adding further transistors to the programmable logic circuit according to any of the first embodiment to the fifth embodiment, one of the source and the drain of each transistor added being connected to the gate of a cut-off transistor. FIG. 17 shows an example of the programmable logic circuit according to the sixth embodiment. The programmable logic circuit 140 includes transistors 36 ₁, 36 ₂, of each of which one of the source and the drain is connected to the gate of one of the cut-off transistors 32 ₁ or 32 ₂ of the programmable logic circuit 140 according to the second example of the first embodiment shown in FIG. 3. The other of the source and the drain of each transistor 36 _(i) (i=1, 2) is connected to a power supply line VL1, and the gate is connected to a power supply line VL2.

(Operation)

In a normal operation, the diode connection of the transistor 36 _(i) (i=1, 2) to the gate of the corresponding cut-off transistor 32 _(i) is achieved by applying a power supply voltage Vdd to the power supply line VL1 and the power supply line VL2 as shown in FIG. 18. The resistance is high from the gate of the transistor 36 _(i) (i=1, 2) to the power supply voltage Vdd since this direction is a backward direction in the diode. Therefore, a considerable bootstrap effect can be obtained. The bootstrap effect here means that if a pulse signal is given to one of the source and the drain of an n-channel MOS transistor, the potential of the gate increases due to capacitive coupling of the gate and the one of the source and the drain. As a result, the application of the voltage Vdd results in a voltage more than Vdd to be applied to the gate of the cut-off transistor 32 _(i). This allows a normal operation to be performed without reducing the operational speed. The cut-off transistor 32 _(i) (i=1, 2) is not turned OFF during normal operations and write operations, and therefore the voltage Vss is not needed to be given to the gate of the cut-off transistor 32 _(i) (i=1, 2). This allows the programmable logic circuit according to the sixth embodiment to be achieved. In a write operation, the voltage applied to the power supply line VL2 is set to turn ON the transistor 36 _(i) (i=1, 2) to reduce the resistance, thereby reducing the bootstrap effect to prevent the breakdown of the gate insulating film of the cut-off transistor 32 _(i) (i=1, 2).

FIGS. 19A and 19B shows simulation results regarding the bootstrap effect. The simulations were performed on the circuit shown in FIG. 19A by means of a simulator “Spice.” In this circuit, one of the source and the drain of the transistor 36 is connected to the gate of the cut-off transistor 32. The lines g₁ and the line g₂ in FIG. 19B show the simulation results of cases where the transistor 36 was removed from the circuit shown in FIG. 19A and a voltage was directly applied to the point A, i.e., the gate of the cut-off transistor 32. If a normal voltage is applied to this circuit, the output signal degrades as indicated by the line g₂ in FIG. 19B. If the voltage applied to the gate of the cut-off transistor 32 is increased (“boost voltage”), the output signal rapidly reaches the voltage Vdd as indicated by the line g₁. The output signal obtained by the circuit with the transistor 36 as shown in FIG. 19A is about the same as that obtained by applying the boost voltage, as can be understood from the line g₃ in the simulation results shown in FIG. 19B. This means that the bootstrap effect prevented the degradation of the output signal.

If the write voltage is not so high, a power supply line VL may serve as the power supply line VL1 and the power supply line VL2 to decrease the number of wiring lines connected to the terminals. This may reduce the area. Diodes 37 _(i) may be used instead of the transistors 36 _(i) (i=1, 2) as shown in FIG. 20.

As described above, a programmable logic circuit can be provided according to the sixth embodiment, the programmable logic circuit operating without increasing power consumption and providing signals that are not degraded in normal operation, the gate insulating film of the cut-off transistor in the programmable logic circuit not being broken down when write pulses are applied.

According to the sixth embodiment, a voltage for writing data to a programmable device and not breaking down a gate insulating film of a transistor can be selected as the program voltage Vpp as in the case of the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

Seventh Embodiment

FIG. 21 shows a programmable logic circuit according to a seventh embodiment. The programmable logic circuit according to the seventh embodiment is obtained by adding transistors 24 ₁, 24 ₂ to the programmable logic circuit according to the third example of the first embodiment shown in FIG. 6, one of the source and the drain of each transistor being connected to the gate of the corresponding cut-off transistor 22 ₁ or 22 ₂. The other of the source and the drain of each transistor 24 _(i) (i=1, 2) is connected to a power supply line VL1, and the gate is connected to a power supply line VL2.

As in the case of the sixth embodiment, a programmable logic circuit can be provided according to the seventh embodiment, the programmable logic circuit operating without increasing power consumption and providing signals that are not degraded in a normal operation, the gate insulating film of the cut-off transistor in the programmable logic circuit not being broken down when write pulses are applied.

According to the seventh embodiment, a voltage for writing data to a programmable device and not breaking down a gate insulating film of a transistor can be selected as the program voltage Vpp as in the case of the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

As in the case of the sixth embodiment, transistors 36 ₁, 36 ₂ may be added to the programmable logic circuit according to the seventh embodiment, one of the source and the drain of each transistor being connected to the gate of the corresponding cut-off transistor 32 ₁ or 32 ₂.

Eighth Embodiment

FIG. 22 shows a programmable logic circuit according to an eighth embodiment. The programmable logic circuit according to the eighth embodiment includes memory transistors MT₁₁-MT₃₃ to serve as programmable devices arranged in a 3×3 matrix with rows and columns. Each memory transistor MT_(ij) (i=1, 2, 3, j=1, 2, 3) is, for example, a MOS transistor, in which the gate insulating film may be to be broken down.

The gates of the memory transistors MT₁₁, MT₁₂, MT₁₃ in the first row are connected to a wiring line n01. The gates of the memory transistors MT₂₁, MT₂₂, MT₂₃ in the second row are connected to a wiring line n02. The gates of the memory transistors MT₃₁, MT₃₂, MT₃₃ in the third row are connected to a wiring line n03.

One of the source and the drain of each of the memory transistors MT₁₁, MT₂₁, MT₃₁ in the first column is connected to a wiring line n04. One of the source and the drain of each of the memory transistors MT₁₂, MT₂₂, MT₃₂ in the second column is connected to a wiring line n05. One of the source and the drain of each of the memory transistors MT₁₃, MT₂₃, MT₃₃ in the third column is connected to a wiring line n06.

Although one of the source and the drain of each memory transistor MT_(ij) is connected to a wiring line in the above case, both of the source and the drain may be connected to the wiring line to perform the same operation.

On terminal of each of the wiring lines n01, n02, n03 is connected to a corresponding one of bit lines BL₁, BL₂, BL₃ via a corresponding one of p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃. The other terminal of each of the wiring lines n01, n02, n03 is connected to the input terminal of a corresponding one of CMOS inverters 34 ₁, 34 ₂, 34 ₃ via a corresponding one of cut-off transistors 32 ₁, 32 ₂, 32 ₃. The breakdown voltage for gate insulating films of the p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃ is equal to or higher than that for the memory transistors MT₁₁-MT₃₃ and the cut-off transistors 32 ₁, 32 ₂, 32 ₃. The gates of the cut-off transistors 32 ₁, 32 ₂, 32 ₃ are connected to a wiring line Nc. Output terminals of the CMOS inverters 34 ₁, 34 ₂, 34 ₃ are connected to output terminals out1, out2, out3 of the programmable logic circuit, respectively. The gates of the p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃ are connected to a wiring line nGHLP.

One terminal of each of the wiring lines n04, n05, n06 is connected to the output terminal of the corresponding one of the CMOS inverters 20 ₁, 20 ₂, 20 ₃ via the corresponding one of the cut-off transistors 22 ₁, 22 ₂, 22 ₃. The other terminals of the wiring lines n04, n05, n06 are connected to word lines WL₁, WL₂, WL₃, respectively. The gates of the cut-off transistors 22 ₁, 22 ₂, 22 ₃ are connected to a wiring line Nbst. Input terminals of the CMOS inverters 20 ₁, 20 ₂, 20 ₃ are connected to input terminals in1, in2, in3 of the programmable logic circuit, respectively.

(Write Method)

A method of writing data to the programmable logic circuit according to the eighth embodiment will be described with reference to FIG. 23, taking, as an example, an operation for writing data to the memory transistor MT₁₁. FIG. 23 shows voltage conditions for writing data to the memory transistor MT₁₁.

First, a program voltage Vpp is applied to the bit line BL₁ to which the gate of the memory transistor MT₁₁ is connected, and a voltage Vhp1 for turning ON the transistors 38 ₁, 38 ₂, 38 ₃ is applied to the wiring line nGHLP to which the gates of the transistor 38 ₁, 38 ₂, 38 ₃ are connected.

If the voltage difference Vpp−Vhp1 is in a level not to break down the gate insulating film of a p-channel MOS transistor, transistors with a breakdown voltage for the gate insulating film similar to that for the memory transistors MT₁₁-MT₃₃ and the cut-off transistors 32 ₁, 32 ₂, 32 ₃ can be used as the p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃.

An operating voltage Vdd for operating CMOS inverters is applied to the input terminal in1 to set the potential of the wiring line n04 to be 0 V, and a voltage Von is applied to the wiring line Nbst to turn ON the cut-off transistors 22 ₁, 22 ₂, 22 ₃. As a result, a voltage of 0 V is applied to the source and the drain of the memory transistor MT₁₁, and the program voltage Vpp is applied to the gate thereof, thereby allowing a write operation to be performed. The operating voltage for the CMOS inverters in a write operation may be a write inhibiting voltage Vinh. In this case, the voltage Vinh is applied to the input terminal in1. The write inhibiting voltage Vinh is between the voltage Vdd and the voltage Vpp, and the voltage difference Vinh-Vss does not break down the memory transistors MT_(ij) and the cut-off transistors 22 _(j), 32 _(i).

The program voltage Vpp is also applied to the gates of the memory transistors MT₁₂, MT₁₃. However, the writing to these transistors can be prevented by applying 0 V to the input terminals in2 and in3 so that the potentials of the wiring lines n05 and n06 become the operating voltage Vdd for the CMOS inverters or the voltage Vinh.

The bit lines BL₂ and BL₃, to which the gates of the memory transistors, on which the write operation is not performed, are brought into a floating state, or the write inhibiting voltage Vinh or the voltage Vss is applied thereto. Thus, the write operation is not performed on these memory transistors, the gates of which are connected to the bit line BL₂ or BL₃.

The input direction in the programmable logic circuit according to the eighth embodiment is preferably the direction of the wiring line to which one of the source and the drain of each of the memory transistor MT₁₁-MT₃₃ is connected, for example the direction along which the wiring lines n04, n05, n06 extend, and the output direction is preferably the direction of the wiring line to which the gate of each of the memory transistors MT₁₁-MT₃₃ is connected, for example the direction along which the wiring lines n01, n02, n03 extend. The reasons for the above is that if the write voltage Vpp is applied to the wiring line (for example n04) to which the source and the drain of a transistor to be programmed (for example MT₁₁) is connected, and 0 V is applied to the gate wiring line, a leakage current may flow to the substrates of other memory transistors (for example MT₂₁, MT₃₁), the sources and the drains of which are connected to the wiring line n04.

(Operation)

A method of operating (reading data from) the programmable logic circuit according to the eighth embodiment will be described with reference to FIG. 24. For simplification, a method of reading data from the memory transistor MT₁₁ will be described below. FIG. 24 shows voltage conditions to read data from the memory transistor MT₁₁.

First, a voltage Vhp2 is applied to the wiring line nGHLP to which the gates of the transistors 38 ₁, 38 ₂, 38 ₃ are connected to turn OFF the transistors 38 ₁, 38 ₂, 38 ₃. This disconnects the bit lines BL₁, BL₂, BL₃ used for the write operation from the memory array including the memory transistors MT₁₁-MT₃₃. A voltage Vbst for turning ON the cut-off transistors 22 ₁-22 ₃, 32 ₁-32 ₃ is applied to the wiring line Nbst and the wiring line Nc. This allows a signal Vread1 inputted to the input terminal in1 to be outputted from the output terminal out1 via the memory transistor MT₁₁. Unless data has been written to the memory transistors other than the memory transistor MT₁₁, signals inputted to the input terminals in2, in3 are not outputted from the output terminals.

According to the eighth embodiment, a voltage for writing data to a memory transistor (programmable device) and not breaking down the gate insulating film of the transistor can be selected as the program voltage Vpp, as in the case of the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

As in the case of the sixth embodiment, the programmable logic circuit according to the eighth embodiment may further include transistors, one of the source and the drain of each of which is connected to the gate of the corresponding one of the cut-off transistor 32 ₁, 32 ₂, 32 ₃.

Furthermore, as in the case of the programmable logic circuit shown in FIG. 21, the programmable logic circuit according to the eighth embodiment may further include transistors, one of the source and the drain of each of which is connected to the gate of the corresponding one of the cut-off transistors 22 ₁, 22 ₂, 22 ₃.

Although an example of memory elements arranged in a 3×3 matrix has been described, the number of memory elements may be increased.

Ninth Embodiment

A programmable logic circuit according to a ninth embodiment will be described with reference to FIGS. 25 to 27. FIG. 25 is a circuit diagram showing a programmable logic circuit according to the ninth embodiment. The programmable logic circuit according to the ninth embodiment is obtained by adding n-channel MOS transistors 26 ₁, 26 ₂, 26 ₃ to respective columns corresponding to the wiring lines n04, n05, n06 of the programmable logic circuit according to the eighth embodiment shown in FIG. 22. The breakdown voltage for gate insulating films of these transistors 26 ₁, 26 ₂, 26 ₃ is higher than or similar to the breakdown voltage for the memory transistors MT₁₁-MT₃₃ and the cut-off transistors 32 ₁, 32 ₂, 32 ₃. One of the source and the drain of each of the transistors 26 ₁, 26 ₂, 26 ₃ is connected to the corresponding one of the wiring lines n04, n05, n06, the other is connected to the corresponding one of the word lines WL1, WL2, WL3, and the gate is connected to a wiring line nGHLN.

In the programmable logic circuit according to the eighth embodiment, a high voltage is applied to the gate of a memory transistor to which data is to be written to cause breakdown of the gate insulating film thereof. The potential of the word line (for example, the word line WL₁) to which the memory transistor (for example, the memory transistor MT₁₁) to which data is written should be reduced immediately in order to prevent the breakdown of the gate insulating films of other memory transistors (for example, memory transistors MT₂₁, MT₃₁), one of the source and the drain of each of which is connected to the same wiring line as that of the memory transistor to which data is written (for example, memory transistor MT₁₁). In order to reduce the potential, the size of the input inverters 20 ₁, 20 ₂, 20 ₃ is increased in the eighth embodiment to draw current rapidly. In order to increase the size of the inverters, however, the size of the n-channel MOS transistors and the p-channel MOS transistors should also be increased.

The n-channel MOS transistors 26 ₁, 26 ₂, 26 ₃ connected to the wiring lines n04, n05, n06 in the ninth embodiment are intended to allow the input inverters 20 ₁, 20 ₂, 20 ₃ to maintain the speed required for reading signals, and to reduce the size and the power consumption in operation.

(Write Method)

A method of writing data to the programmable logic circuit according to the ninth embodiment will be described with reference to FIG. 26. FIG. 26 shows voltage conditions for writing data to the memory transistor MT₁₁.

First, a program voltage Vpp is applied to the bit line BL₁ to which the gate of the memory transistor MT₁₁ is connected, and a voltage Vhp1 is applied to the wiring line nGHLP to which the gates of the p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃ are connected to turn ON the transistors 38 ₁, 38 ₂, 38 ₃. A voltage of 0 V is applied to the word line WL₁ to set the potential of the wiring line n04 to be 0 V, and a voltage Vhn1 is applied to the wiring line nGHLN to turn ON the transistors 26 ₁, 26 ₂, 26 ₃. This applies the voltage 0 V to the source and the drain of the memory transistor MT₁₁, and the program voltage Vpp to the gate thereof to allow a write operation to be performed.

If the voltage difference Vpp−Vhn1 would not break down the gate insulating films of n-channel MOS transistors, transistors with about the same breakdown voltage for gate insulating films as that of the memory transistors MT₁₁-MT₃₃ and the cut-off transistors 32 ₁, 32 ₂, 32 ₃ can be used as the n-channel MOS transistors 26 ₁, 26 ₂, 26 ₃. Although the program voltage Vpp is also applied to the gates of the memory transistors MT₁₂, MT₁₃, data would not be written to these transistors if a write inhibiting voltage Vinh is applied to the word line WL₂ and the word line WL₃.

The cut-off transistors 22 ₁, 22 ₂, 22 ₃ connected to the wiring lines n04, no5, n06 connecting to the transistors 26 ₁, 26 ₂, 26 ₃ are turned OFF to block signals from the input terminals in1, in2, in3. The write operation is not performed on the memory transistors MT₂₁-MT₃₃, by bringing into a floating state the bit lines BL₂, BL₃, to which the gates of these memory transistors MT₂₁-MT₃₃ are connected, or a write inhibiting voltage Vinh or a voltage Vss is applied to the bit lines BL₂, BL₃.

(Operation)

A method of operating (reading data from) the programmable logic circuit according to the ninth embodiment will be described with reference to FIG. 27. FIG. 27 shows voltage conditions for reading data from the memory transistors MT₁₁ after writing data to the memory transistor MT₁₁.

First, a voltage Vhp2 is applied to the wiring line nGHLP to which the gates of the transistors 38 ₁, 38 ₂, 38 ₃ are connected to turn OFF the transistors 38 ₁, 38 ₂, 38 ₃. As a result, the bit lines BL₁-BL₃ used for the write operation are disconnected from the memory array. Similarly, a voltage Vhn2 is applied to the transistors 26 ₁-26 ₃ to turn OFF these transistors. As a result, the word lines WL₁-WL₃ used for the write operation are disconnected from the memory array.

A voltage Vbst is applied to the wiring lines Nbst, Nc to turn ON the cut-off transistors 22 ₁-22 ₃, 32 ₁-32 ₃. As a result, a signal Vread1 inputted to the input terminal in1 passes through the memory transistor MT₁₁ and is outputted from the output terminal out1. Input signals to the input terminals in2, in3 are not outputted from the output terminals out2, out3 unless the memory transistors MT₂₁, MT₃₁ are programmed.

Diodes 28 ₁, 28 ₂, 28 ₃ may replace the transistors 26 ₁, 26 ₂, 26 ₃ as shown in FIG. 28.

According to the ninth embodiment, a voltage for writing data to a memory transistor (programmable device) and not breaking down the gate insulating film of a transistor can be selected as the program voltage Vpp as in the case of the first embodiment. This allows direct amplification of signals on wiring lines to which the program voltage Vpp is applied, thereby allowing a high speed operation.

As in the case of the sixth embodiment, the programmable logic circuit according to the ninth embodiment may further include transistors, one of the source and the drain of each of which is connected to the gate of the corresponding one of the cut-off transistors 32 ₁, 32 ₂, 32 ₃.

Furthermore, as in the case of the programmable logic circuit shown in FIG. 21, the programmable logic circuit according to the ninth embodiment may further include transistors, one of the source and the drain of each of which is connected to the gate of the corresponding one of the cut-off transistors 22 ₁, 22 ₂, 22 ₃.

The p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃ of the eighth embodiment and the ninth embodiment may be replaced with n-channel MOS transistors or diodes D₁, D₂, D₃. In the case of n-channel MOS transistors, the gate voltage for n-channel MOS transistors in write operation is higher than program voltage Vpp by the threshold voltage of n-channel MOS transistor. FIG. 29 shows an example of a programmable logic circuit in which the p-channel MOS transistors 38 ₁, 38 ₂, 38 ₃ are replaced with diodes D₁, D₂, D₃ in the programmable logic circuit according to the eighth embodiment shown in FIG. 22. The replacement of the p-channel MOS transistors with the n-channel MOS transistors or the diodes has an effect of reducing area.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

1. A programmable logic circuit comprising: a plurality of first wiring lines; a plurality of second wiring lines intersecting with the first wiring lines in intersecting areas; a plurality of third wiring lines; a plurality of cells provided in the intersecting areas, at least one of the cells including a first transistor with a source, a drain, and a gate, and a programmable device with a first terminal and a second terminal, the first terminal connecting to one of the source and the drain of the first transistor, the second terminal being connected to one of the second wiring lines, the other of the source and the drain of the first transistor being connected to one of the first wiring lines, and the gate of the first transistor being connected to one of the third wiring lines; a plurality of first cut-off transistors each including a source and a drain, one of the source and the drain being connected to the one of the second wiring lines; a plurality of first CMOS inverters corresponding to the first cut-off transistors, each of the first CMOS inverters including an input terminal, the input terminal being connected to the other of the source and the drain of the corresponding one of the first cut-off transistors. 2.-20. (canceled) 